With the availability of heat engines utilizing a fully regenerative phase-change thermodynamic cycle, it is possible to convert the thermal energy contained in a uniform-temperature environment directly to mechanical or electrical energy (see PCT/US11/41289 Thermal Engine Capable of Utilizing Low-Temperature Sources of Heat, inventor Neil Tice, and U.S. Pat. No. 7,816,601 Device and method for converting thermal energy into electrical energy, inventor David Reginald Carver) without the need for an additional reservoir of a cooler temperature in which to dump waste heat. The key component of this type of engine is a working medium consisting of a material which undergoes a first phase change while absorbing latent heat of transformation, then undergoes a return phase change to complete the cycle.
Any phase change from one state to another, including from one solid state to another solid state, for example, can be used to convert heat to another form of energy. Many of the potential phase transitions involve a change from one fluid state to another fluid state. Fluid in this sense means the ability to flow, and is not restricted to liquids. Examples may include liquid to vapor, liquid to liquid, and vapor to vapor. Additional possibilities may involve compound mediums, such as a liquid with solid particles blended into a slurry, with one or both undergoing a phase change which results in a change in volume or pressure. Any compound which undergoes a change in volume or pressure as a result of one or more phase changes, and remains fluid in all relevant states, is a candidate for use in the present invention.
One inefficiency experienced in heat engines, in general, is that in the transition from the high-enthalpy gas to the low-enthalpy liquid state and the return transition from gas to liquid state (in, for example, an engine driven by the liquid to gas transition), either the work performed by the transition from low enthalpy to high enthalpy is not captured, or the latent heat energy released by the transition from high enthalpy to low enthalpy is wasted. Efficiencies may be gained by utilizing the latent heat expressed during the high- to low-enthalpy phase change and capturing the work output of the low- to high-enthalpy phase change.